The present invention relates to a process for preparing a polyolefin foam in a mold from expandable polyolefin beads. More particularly, the invention relates to a process for preparing a well-fused and less-shrinkable polyolefin foam having excellent appearance by controlling the ratio of the peak expansion rate of beads in a pre-expansion stage to the expansion rate of beads to be molded within a definite predetermined range to made the best of the expandability of pre-expanded beads.
Polyolefin beads are usually pre-expanded by incorporating a foaming agent therein and heating them in a pre-expansion tank to produce pre-expanded beads. However, polyolefin resins differ remarkably from polystyrene resins in gas permeability, viscoelasticity of melt, rigidity of resin, and the like so that if a polyolefin resin is expanded to a high bulk in a single pre-expansion, the pre-expanded foam cells are crushed in a few seconds or hours after removal from the pre-expansion tank to give only shrunken pre-expanded beads. Thus, it is very difficult to control the heat-molding conditions. Therefore, even if the beads are pre-expanded to a high bulk or expansion rate, the expansion rate of beads some time after removal from the mold is so low that if used for a post-expansion, the beads give a hard molding having only a low expansion rate.
As an attempt to overcome the above-mentioned disadvantages of the conventional processes for preparation of a polyolefin foam from beads and to provide an industrial process, Japanese Examined Patent Publication No. 22951/1977 and Japanese Unexamined Patent Publication No. 128065/1974 disclose a process wherein a pressurized inorganic gas is introduced into the pre-expanded beads under pressure to recover the loss of volume due to shrinkage and impart additional expandability thereto before submitting them to a post-expansion stage. More recently, with the progress of research in polyolefin foams, several new processes have been proposed. Those processes include a process in which various molding gases are added to the pre-expanded beads, a process in which pre-expanded beads are compressed and filled into a mold (Japanese Examined Patent Publication No. 33996/1978), a process in which a molded foam is subjected to a complicated temperature history treatment without addition of a molding gas (Japanese Examined Patent Publication No. 7816/1980), and a process in which a mixture of water vapor with a particular gas are used in molding step (Japanese Unexamined Patent Publication No. 88566/1976).
However, those prior art processes have their own drawbacks and involve many problems that must be solved and equipment-wise limitations. For example, in the process involving an additional supply of molding gas, the beads must be molded within a relatively short period of time during which more than a required amount of the foaming agent is still present in the expandable beads and it is necessary to link the step of supplying the molding gas with the molding step in a continuous arrangement. The process in which the pre-expanded beads are compressed and packed in a mold (Japanese Examined Patent Publication No. 33996/1978) is designed to impart additional expansibility to beads by subjecting them to a compression load. In such process, however, it is necessary to make a special arrangement for feeding the compressed beads into the mold and, also, to synchronize a compression device with a molding machine.
As regards the process wherein the pre-expanded beads are used as they are in the molding step without an additional supply of molding gas, it is necessary, as described in the aforementioned Japanese Examined Patent Publication No. 7816/1980, that after application molding heat, the molded foam is subjected to a temperature history such that it is first cooled to a temperature below the softening point of the resin or nomal temperature, then heated to the softening temperature of the resin or a temperature lower than the softening temperature by 40.degree. C. and finally cooled gradually. Such a temperature history can be achieved only by complicated temperature and time control and with a complicated equipment.
The process in which a gaseous mixture of water vapor with a foaming gas are used in the heat molding of pre-expanded beads is intended to impart an expandability to the beads at the heat molding stage. The process, however, additionally requires a mixed gas generator, which complicates the molding process.
The present invention can provide a process for preparing a polyolefin foam which is evenly fused to the interior and is less liable to shrink or deform by employing expandable polyolefin beads capable of expanding to a suitable extent in the heat molding.